1. Field of the Invention
The present invention relates to a phosphor, particularly, a silicate phosphor capable of being widely used for a display device such as plasma display panel or the like, to devices of lighting systems and the like such as thin tube type fluorescent lamp and the like, electronic device and various articles using phosphors, to a method for producing the phosphor and its precursor, and to a display device.
2. Description of Related Art
A phosphor is a material for converting the energy of an exciting line (ultraviolet ray, visible light, infrared ray, heat ray, electron ray, X-ray, radioactive ray or the like) into a light (ultraviolet ray, visible light, infrared ray or the like) by irradiating the exciting line. Phosphors are used for various articles such as fluorescent paints, ashtrays, stationeries, outdoor products, guide plates, inducing materials, indicators such as safety signs and the like, and the like. Besides the above, phosphors are also used for various devices such as electron tubes, fluorescent display tubes, electroluminescence panels, scintillation detectors, X-ray image intensifiers, thermoluminescence dosimeters, imaging plates and the like, and for display devices such as field emission display (FED), plasma display panel (PDP) and the like (See for example, “Phosphor Handbook”, Phosphor Research Society Edition, Ohmsha, Ltd.).
Among the above-described display devices, particularly, the PDP is capable of making a screen large and thin, and thereby, it attracts attention as a flat-panel display which can take the place of a cathode-ray tube (CRT). A PDP has two glass substrates comprising electrodes, and a number of micro discharge cells (hereinafter, referred to as “cells”) formed by partition walls provided between the substrates. On the side face and the bottom face of a partition wall which surrounds each cell (one glass substrate), a phosphor layer that emits a light of red, green, blue or the like is provided. Each cell is formed in a predetermined shape by the partition walls, arranged regularly on the substrate, and discharge gas using Xe, Ne or the like as principal component is charged therein. The cell is for suppressing spread of discharge in a certain region. When voltage is applied between the electrodes, an ultraviolet ray due to the discharge gas is generated, and thereby, the phosphors are excited and emit a visible light. Desired information can be displayed in full color by selectively discharging the cell or a part of the cell.
Presently, as phosphors mainly used for a PDP, there are (Y, Gd)BO3:Eu (Red), Zn2SiO4:Mn (Green), BaMgAl10O17:Eu (Blue) and the like. These phosphors are generally produced by a solid phase method. The solid phase method is a method for obtaining a phosphor according to a solid phase reaction by mixing a predetermined amount of a compound including elements forming a ground material of the phosphor with a predetermined amount of a compound including activator elements, and calcining the mixture at a predetermined temperature (See “Phosphor Handbook”). The shape of individual particle of the phosphor obtained according to such a method is usually flat shape or irregular multifaceted shape.
Incidentally, with displays such as PDP and the like, improvement of brightness, smooth movie display and the like are required. Thereby, it is considered to improve the luminescence intensity of phosphors in order to enhance the brightness. Particularly, a green phosphor has high visibility, so that it is important to enhance the luminescence intensity of the green phosphor in order to improve the white brightness. On the other hand, in order to achieve smooth movie display, information has to be displayed in sequence per extremely short unit of time, so that phosphors having short persistence time are required.
However, the above-described silicate phosphor such as Zn2SiO4:Mn (Green) or the like has a long persistence time, so that there is a possibility that it generates afterimage or flickering of image, or the like when the next new information is displayed. Therefore, as for the silicate phosphor such as Zn2SiO4:Mn or the like, it is required to improve the luminescence intensity, and moreover, to reduce the persistence time thereof.
Then, it is considered to improve the luminescence intensity, and moreover, to reduce the persistence time by producing Zn2SiO4:Mn having high purity. However, with the solid phase method in earlier technology, since it is solid phase reaction, there remain nonreactive excessive impurities, excess soluble salts generated by reaction, and the like. Therefore, it is difficult to obtain a phosphor having stoichiometrically high purity. Furthermore, although the luminescence intensity, the persistence time or the like of an inorganic phosphor can be generally controlled by changing the amount of activators, with the solid phase method, it is difficult to properly control the amount of these in the composition.
Further, recently, along with achievement of higher definition of a display such as PDP or the like, cells have been miniaturized. In order to obtain predetermined brightness, not less than a certain amount of phosphors must be filled in every phosphor layer provided in each cell. However, if the particle shape is flat shape or irregular multifaceted shape as mentioned above, the phosphor layers become thick by that shape. When the thickness of the phosphor layers is thick, not only scattering of lights emitted from the phosphors become large, but also the discharge space becomes narrow, so that it becomes impossible to generate an ultraviolet ray sufficient to excite the phosphors. As mentioned before, particularly, the green phosphor has high visibility, so that it is required to improve the brightness of green cells.
Therefore, in order to fill up a green phosphor in a phosphor layer thickly, for instance, a spherical phosphor (Zn2SiO4:Mn) is produced by using spherical or approximately spherical polymethylsilsesquioxane as a base substance (for example, see Japanese Patent Application Laid-Open Publication No. 9-278446; hereinafter, referred to as Patent Document 1).
However, although finer phosphor particles are required in accordance with the miniaturization of the cell structure of a PDP, only a method of obtaining a spherical phosphor which has comparatively large particle size (4.5 μm to 2 μm) according to the solid phase method is disclosed in Patent Document 1. Since the method disclosed in Patent Document 1 is the solid phase method, it is required to perform a step such as crushing or the like when producing phosphor of finer particles. Therefore, there is a possibility that the shape of the phosphor particles becomes not uniform. Further, there is a case such that crystal distortion occurs at the time of crushing, and thereby that the luminescence intensity deteriorates. Furthermore, since powders as raw materials are mixed according to a dry system, it is difficult to control the composition of constituting elements uniform. Therefore, it is difficult to diffuse and mix activators uniformly in center nuclei particles of phosphors, so that there is a possibility that sufficient luminescence intensity cannot be obtained.